Olefinic hydrocarbons are employed as starting materials in the hydroformylation, or oxo, process, for the eventual manufacture of numerous valuable products, e.g., alcohols, esters and ethers derived therefrom, aldehydes, and acids. In many of these end uses, linear or lightly branched hydrocarbon chains have advantages compared with more heavily branched chains.
In the oxo process itself, moreover, olefins with heavily branched chains are less reactive than those with linear or lightly branched structures and, for a given degree of branching, certain isomers are less reactive than others.
Olefinic feedstocks, especially in the C.sub.4 to C.sub.20, and more particularly in the C.sub.6 to C.sub.15 range, are frequently produced by oligomerization of lower molecular weight original starting materials, a process that, because of rearrangements that take place during the reaction, may produce an undesirably high proportion of multiply branched olefins, even if the original materials are linear. Also, the locations of the branches, at sites close to each other on the hydrocarbon chain, or in the central region of the chain, or both, resulting from the oligomerization further reduce the reactivity of the molecules in the oxo reaction.
There are other areas in which a less highly branched hydrocarbon has advantages; these include the alkylation of aromatic hydrocarbons by reaction with olefins in the manufacture of surfactants and polyolefin stabilizers.
There is accordingly a need to provide a method to produce an olefin oligomer having a reduced degree of branching of a hydrocarbon material.
U.S. Pat. No. 5,284,989 (Apelian, et al, assigned to Mobil Oil Corporation) describes the use of a medium pore size shape-selective acid crystalline zeolite in the catalytic oligomerization of olefinic hydrocarbons, and discusses the factors influencing the linearity or degree of branching of the products. Acid activity at the zeolite particle surface is said to favour the production of branched products, and reference is made to de-alumination of zeolite surfaces to reduce surface acidity, or the ratio of surface acidity to intracrystalline acid site activity. Other reduction methods mentioned in an extensive prior art review in the patent include the use of bulky amines to inactivate acid sites; the invention to which the patent is directed is the use of a dicarboxylic acid to inactivate the surface acid sites.
In U.S. Pat. No. 5,250,484 (Beck et al., also assigned to Mobil), surface acidity is reduced by contacting the catalyst with an ammonia-borane solution and calcining to form an inactive ceramic layer on the surface. In U.S. Pat. No. 4,788,374 (Chu et al., also assigned to Mobil), surface acidity is reduced by forming a silica shell on a metallosilicate core by crystallizing silica on the surface of the core in the presence of fluoride.